1. Field of the Invention
The present invention relates to a golf ball. More particularly, the present invention relates to a golf ball comprising a cover having a small thickness.
2. Description of the Related Art
General golf balls excluding golf balls manufactured for a driving range comprise cores and covers. The core is formed by a single solid rubber layer, two or more solid rubber layers, a solid rubber layer and a synthetic resin layer or the like.
It is important for golf ball manufactures to manufacture a uniform golf ball. There have variously been proposed a method of manufacturing a golf ball having a multilayer structure (Japanese Laid-Open Patent Publications Nos. Hei 9-285565 (1997/285565) 2000-330 and 2000-5343).
Examples of characteristics required for a golf ball by a golf player include a long flight distance, an excellent spin performance and a soft hitting feeling. In hitting with a driver, the golf player attaches importance to the flight distance. In hitting with a short iron, the golf player attaches importance to the spin performance and the hitting feeling. In order to correspond to various characteristics which are required, there has been proposed a golf ball comprising a cover having a small thickness.
A golf ball has approximately 200 to 550 dimples on a surface thereof. The role of the dimples resides in one aspect that such dimples disturb an air stream around the golf ball during the flight to accelerate the transition of a turbulent flow at a boundary layer, thereby causing a turbulent flow separation. The acceleration of the transition of the turbulent flow causes a separating point of air from the golf ball to be shifted backward so that a drag coefficient (Cd) is reduced, resulting in an increase in the flight distance of the golf ball. The acceleration of the transition of the turbulent flow increases a differentia between upper and lower separating points of the golf ball which is caused by back spin. Consequently, the lift action on the golf ball is increased. The dimple effect greatly depends on the volume of the dimple.
The thickness of the cover provided under the dimple is smaller than that of the cover of a land portion thereof (which will be hereinafter referred to as a “nominal thickness”). Usually, the depth of the dimple is more than 0.2 mm. In the case of a golf ball having a nominal thickness of 1.2 mm or less, the thickness of the cover provided under the dimple is extremely small. In some cases in which the golf ball is repetitively hit, the dimple acts as the starting point of a crack and the cover is thus broken.
If the nominal thickness is less than the depth of the dimple, the core is exposed to the bottom portion of the dimple. Also in the case in which the nominal thickness is greater than the depth of the dimple, there is a possibility that the core might be exposed to the bottom portion of the dimple if the eccentricity of the core (the center of the core is shifted from that of the golf ball) is caused. In a golf ball having a small nominal thickness, the core is apt to be exposed due to eccentricity. The durability of the golf ball remarkably deteriorates due to the exposure of the core. The exposure of the core decreases the volume of the dimple so that the dimple effect is reduced. Also, the exposure of the core is not desirable for external appearances.
In cover molding, a core is put in a mold. A molten cover material flows in a gap between the core and the cavity surface of the mold. The cavity surface of the mold is provided with a projection having a shape obtained by inverting the shape of a dimple. In the case in which a cover having a small nominal thickness is to be formed, the flow of the cover material is hindered by the projection because the distance between the projection and the core is extremely small. Thus, it is hard to form a cover having a small nominal thickness.